1. Field of the Invention
The present invention relates to a semiconductor device having high withstanding voltage and high driving performance for a metal oxide semiconductor (MOS) transistor, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
FIG. 4 is a cross-section illustrating an example of a conventional MOS transistor having a high withstanding voltage and a high driving performance. A semiconductor device 200 illustrated in FIG. 4 includes a semiconductor substrate 1, a first conductivity type well region 2, a second conductivity type low concentration drain region 3, a second conductivity type high concentration source region 4, a second conductivity type high concentration drain region 5, a gate oxide film 6, a channel formation region 7, a polycrystalline silicon gate electrode 8, a field oxide film 9, a protective oxide film 16, a source electrode 10, and a drain electrode 11. A feature of this structure resides in that the second conductivity type low concentration drain region 3 is formed between the channel formation region 7 and the second conductivity type high concentration drain region 5 to thereby attain high with standing voltage while the channel formation region 7 and the second conductivity type high concentration source region 4 are brought into direct contact with each other to thereby attain high driving performance.
In the case of employing the structure described above, ion implantation for forming the second conductivity type high concentration source region 4 and the second conductivity type high concentration drain region 5 is performed over the gate oxide film 6 with the polycrystalline silicon gate electrode 8 being used as a mask. Here, in order to attain not only high source-drain withstanding voltage but also high gate-source withstanding voltage, it is necessary to increase a thickness of the gate oxide film 6, which may cause a problem that stable ion implantation for forming the second conductivity type high concentration source region 4 and the second conductivity type high concentration drain region 5 cannot be performed depending on kinds of impurities to be used and capability of an ion implantation apparatus. In view of the problem, there is known a method of removing the gate oxide film 6 through etching after the polycrystalline silicon gate electrode 8 has been formed before ion implantation for forming the second conductivity type high concentration source region 4 and the second conductivity type high concentration drain region 5 is performed. However, a source-side region 12 of the gate oxide film 6 is also side-etched during the etching, resulting in a structure having a region in which a part of the gate oxide film 6 located below the polycrystalline silicon gate electrode 8 is removed, as a semiconductor device 201 illustrated in FIG. 5. As in the case of the semiconductor device 201, if the gate oxide film 6 formed on the channel formation region 7 is removed even partially so that the protective oxide film 16 occupies the side-etched portion, performance of the semiconductor device is significantly deteriorated. Accordingly, an upper limit is placed on the thickness of the gate oxide film 6 with which the structure of the semiconductor device 200 may be obtained. To eliminate the upper limit, there has been proposed a method in which a source field oxide film is formed also on a source side of the channel formation region, and an impurity concentration of a field region below the source field oxide film is set higher than an impurity concentration of the second conductivity type low concentration drain region 3 (see, for example, JP 2002-208694 A).
With the method described above, a high driving performance may be attained even in a MOS transistor using a thick gate oxide film for a high withstanding voltage. However, because the impurity implantation into the region below the source field oxide film is performed before the formation of the field oxide film, a problem arises in which the film quality thereof deteriorates if the impurity concentration of the region below the source field oxide film becomes too high, also causing deterioration of characteristics of the transistor.